1. Field of the Invention
The present invention relates to a capacitive load driving circuit, a droplet ejection device, a droplet ejection unit and an inkjet head driving circuit, and more particularly relates to a capacitive load driving circuit, droplet ejection device, droplet ejection unit and inkjet head driving circuit for driving capacitive loads.
2. Description of the Related Art
Heretofore, inkjet head driving circuits have caused ink droplets to be ejected from nozzles of inkjet heads, which nozzles are provided in correspondence with piezoelectric actuators, by outputting analog driving signals to the piezoelectric actuators so as to discharge the ink droplets from the nozzles. In such an inkjet head driving circuit, an analog amplification circuit is employed, and an analog driving signal which has been power-amplified by the analog amplification circuit is outputted to the piezoelectric actuators. However, analog amplification circuits have a drawback in that power supply efficiency is poor and they tend to generate heat during power amplification. Consequently, when a number of piezoelectric actuators are driven at the same time, a lot of heat is generated and there is a risk of heat damage to the driving circuit itself.
Further, it has been necessary to additionally mount radiators at inkjet head driving circuits in order to dissipate the heat generated by the analog amplification circuits.
Further yet, because piezoelectric actuators are capacitive elements, there is a problem in that when the number of piezoelectric actuators that are being driven at the same time is large, the waveform of a driving signal being inputted to the piezoelectric actuators is degraded, and when the number of piezoelectric actuators being driven at the same time is small, there is a lot of ringing in the waveform of the driving signal.
As a technology for solving these problems, a technique of correcting a voltage of a driving signal at an inkjet head driving circuit in accordance with an environmental temperature of an inkjet printer has been disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-20203). According to the technology of JP-A No. 11-20203, the voltage of an analog driving signal outputted from a driving circuit is corrected such that the voltage is lower when the environmental temperature is higher and the voltage is higher when the environmental temperature is lower. In addition, a structure thereof employs a thermistor which has a characteristic of resistance falling when environmental temperature rises, so as to prevent thermal runaway of the driving circuit. As a result, a danger of thermal damage to the driving circuit can be avoided.
Further, in a technology of JP-A No. 2000-117980 relating to inkjet head analog amplification circuits, rather than bipolar transistors, MOSFETs are used as transistors included in the analog amplification circuits. Thus, the danger of thermal damage to the driving circuits due to heat generation is avoided.
JP-A No. 2000-117980 also discloses a technique for generating analog driving signals to drive piezoelectric actuators with large load capacitances without malfunctioning. In this technology of JP-A No. 2000-117980, negative feedback is applied to terminal voltages of the piezoelectric actuators in order to maintain terminal voltages of the piezoelectric actuator at a predetermined bias voltage. As a result, it is possible to compensate for degradation of the waveform of the analog driving signal that is inputted to the piezoelectric actuators.
With the conventional technologies described above, it is possible to avoid the risk of thermal damage to a driving circuit and to compensate for degradation of a waveform of an analog driving signal. However, none of the related technologies described above suppresses heating of the driving circuit itself. Therefore, it is still necessary to mount a radiator in order to dissipate heat generated in an analog amplification circuit.
Now, D-class amplification circuits which use pulse width modulation are known as amplification circuits in which power supply efficiency is good and heat generation temperatures are low. Whereas analog amplification circuits are of types which utilize linear amplification operations of transistors and the like, D-class amplification circuits are circuits which perform amplification by using digital techniques, meaning switching operations, utilizing variations in average output provided by ratios of on and off states of power sources. More specifically, at a D-class amplification circuit, input signals are pulse width-modulated to digital signals, power amplification is performed on the digital signals, the digital signals are demodulated back to analog driving signals after power amplification, and the analog driving signals are outputted. Thus, it is possible to perform amplification more efficiently than with an analog amplification circuit, and power consumption and heat generation are lower. Hence, in recent years, D-class amplification circuits have been particularly employed in audio circuits.
Employing such a D-class amplification circuit in place of an analog amplification circuit in an inkjet printer driving circuit has been considered as a method for suppressing the generation of heat in the driving circuit. However, there are a number of problems with employing D-class amplification circuits in inkjet head driving circuits, and this has not yet been realized.
To be specific, the load of an audio circuit is a speaker, so the load can be regarded as a substantially resistive load, and there is little load variation. In contrast, a piezoelectric actuator is a capacitive load, and the load varies in accordance with the number of actuators being used at the same time, which is problematic.
Furthermore, because a D-class amplification circuit performs power amplification after an input signal has been converted to a digital signal, a low-pass filter (LPF) is provided in order to return the power-amplified signal to an analog signal. The low-pass filter is structured with an inductor and a capacitor. Therefore, when capacitive elements such as piezoelectric actuators are being driven, a cutoff frequency of the LPF will vary in accordance with load variations due to variations in the number of piezoelectric actuators being driven at one time, which is problematic.
Moreover, for audio signals, signals in a frequency range of up to 10 kHz, possibly 20 kHz, are used. However, the range of driving signals of piezoelectric actuators is 100s of kHz. Accordingly, a sampling frequency of 5 MHz to 10 MHz is necessary for application of a D-class amplification circuit to an inkjet head driving circuit, but it is difficult to perform rapid switching operations at these frequencies in a D-class amplification circuit.
Because of these problems, it has been difficult to apply D-class amplification circuits to inkjet head driving circuits.